The present invention concerns preferred methods and apparatus for transdermal delivery or transport of therapeutic agents, typically through electrotransport. Herein the term "electrotransport" is used to refer to methods and apparatus for transdermal delivery of therapeutic agents, whether charged or uncharged, by means of an applied electromotive force to electrolyte-containing reservoir. The particular therapeutic agent being delivered may be charged or uncharged, depending upon the particular method chosen. When the therapeutic species being delivered is charged, the process is referred to as iontophoresis. When the therapeutic species delivered is uncharged, it may be considered delivered by means of electro-osmosis techniques or other electrokinetic phenomenon such as electrohydrokinesis, electro-convection or electrically-induced osmosis. In general, these latter electrokinetic delivery processes of uncharged species into a tissue result from the migration of solvent, in which the uncharged species is dissolved, as a result of the application of electromotive force to the electrolyte reservoir. Of course during the process, some transport of charged species will take place as well.
In general, iontophoresis is an introduction, by means of electric current, of ions of soluble salts into the tissues of the body. More specifically, iontophoresis is a process and technique which involves the transfer of ionic (charged) species into a tissue (for example through the skin of a patient) by the passage of a electric current through an electrolyte solution containing ionic molecules to be delivered (or precursors for those ions), upon application of an appropriate electrode polarity. That is, ions are transferred into the tissue, from an electrolyte reservoir, by application of electromotive force to the electrolyte reservoir.
Much of the discussion herein will focus on techniques for iontophoresis, and apparatus therefor. However, the methods and apparatus will be understood to be applicable to electrotransport generally, including electrokinetic phenomena involving transport of an uncharged therapeutic species. A reason for this, is that such phenomena generally involve the transport of some charged species, which is accompanied by the desired movement of an uncharged therapeutic species.
Assume, for example, that the patient to receive the therapeutic ion treatment is a human and the medication is to be transferred through the skin. Through iontophoresis, either positively charged drugs (medication) or negatively charged drugs (medication) can be readily transported through the skin and into the patient. This is done by setting up an appropriate potential between two electrode systems (anode and cathode) in electrical contact with the skin. If a positively charged drug is to be delivered through the skin, an appropriate electromotive force can be generated by orienting the positively charged drug species at a reservoir associated with the anode. Similarly, if the ion to be transferred across the skin is negatively charged, appropriate electromotive force can be generated by positioning the drug in a reservoir at the cathode. Of course, a single system can be utilized to transfer both positively charged and negatively charged drugs into a patient at a given time; and, more than one cathodic drug and/or more than one anodic drug may be delivered from a single system during a selected operation. For general discussions of iontophoresis see: Phipps, J. B. et al; "Transport of Ionic Species Through Skin"; Solid State Ionics; Vol. 28-30, p. 1778-1783 (1988); Phipps, J. B., et al; "Iontophoretic Delivery of Model Inorganic and Drug Ions"; J. Pharm. Sciences; Vol. 78, No. 5, p. 365-369 (May 1989); and, Chien, Y. W. et al; "Iontophoretic Delivery of Drugs: Fundamentals, Developments and Biomedical Applications"; J. Controlled Release, Vol. 7, p. 1-24 (1988). The disclosures of these three references are incorporated herein by reference.
Electrotransport processes, including iontophoresis, have found a wide variety of therapeutic applications. Such applications have sometimes involved the delivery of ionic drugs, i.e., charged organic medications or therapeutic metal ions. Applications have involved both treatments of conditions and also diagnostics. For example, iontophoresis techniques have been utilized to deliver pilocarpine, a substance utilized in the diagnosis of cystic fibrosis. It has also been utilized to deliver hyaluronidase, for treatment of scleroderma and lymphedema. It has further been utilized for allergy testing, delivery of metallic ions for treatment of fungal infections, venereal diseases, ulcers, bursitis, and myopathies; delivery of vasodilators; and, for delivery of anesthetics and steroids. See for example Chien, Y. W., et al., supra.
A wide variety of iontophoresis devices are presently known. See for example: Phipps et al., U.S. Pat. No. 4,744,788; Phipps et al., U.S. Pat. No. 4,747,819; Tapper et al., European Patent Application Publication No. 0318776; Jacobsen et al., European Patent Application Publication No. 0299631; Petelenz et al., U.S. Pat. No. 4,752,285; Sanderson et al., U.S. Pat. No. 4,722,726; and Parsi, E. J., U.S. Pat. No. 4,731,049. The disclosures of these seven references are incorporated herein by references.
In typical, conventional, electrotransport devices, for example iontophoresis devices, two electrodes are generally used. Both electrodes are disposed so as to be an intimate electrical contact with some portion (typically skin) of the subject (human or animal) typically by means of two remote electrolyte-containing reservoirs, between which current passes as it moves between the skin and the electrodes. One electrode, generally referred to herein as the "active" electrode, is the electrode from which the substance (medicament, drug precursor or drug) is delivered or driven into the body by application of the electromotive force. The other electrode, typically referred to as an "indifferent" or "ground" electrode, serves to close the electrical circuit through the body. In some instances both electrodes may be "active", i.e. drugs may be delivered from both. In such cases each electrode will serve as the "companion", "indifferent", "remote" or "ground" electrode, to the other. That is, classification of an electrode as "active" or "indifferent" is done by reference to a particular material being delivered. Herein the term electrode, or variants thereof, when used in this context refers to an electrically conductive member, through which a current passes during operation.
If the electrotransport method is iontophoresis, generally the active electrode includes the therapeutic species as a charged ion, or a precursor for the charged ion, and the transport occurs through application of the electromotive force to the charged therapeutic species. If other electrotransport phenomenon are involved, the therapeutic species will be delivered in an uncharged form, transfer being motivated, however, by electromotive force. For example, the applied current may induce movement of a non-therapeutic species, which carries with it water into the subject. The water may have dissolved therein the therapeutic species. Thus, electrotransport of the non-therapeutic charged species induces movement of the therapeutic but non-charged species.
In conjunction with the patient's skin in electrical communication with the electrodes, the circuit is completed by connection of the two electrodes to a source of electrical energy as a direct current; for example, a battery or a source of appropriately modified alternating current. As an example, if the ionic substance to be driven to the body is positively charged, then the positive electrode (the anode) will be the active electrode and the negative electrode (the cathode) will serve to complete the circuit. If the ionic substance to be delivered is negatively charged, then the negative electrode (cathode) will be the active electrode and the positive electrode (anode) will be the indifferent electrode.
Again, electrotransport devices generally require a reservoir as a source of the species (or a precursor of such species) which is to be moved or introduced into the body. If the device is an iontophoresis device, in general the reservoir is a pool of electrolyte solution, for example an aqueous electrolyte solution or a hydrophilic, electrolyte-containing, gel or gel matrix or absorbent material. Such drug reservoirs, when electrically connected to the anode or the cathode of an iontophoresis device, provide a source of one or more ionic species for electrotransport.
Herein, for electrotransport processes, the reservoir associated with the active electrode will be referred to as the "active electrode reservoir." It is this reservoir which includes the "target species" or "therapeutic species," for transport; if as a charged species more specifically for electrotransport. The reservoir associated with the other electrode will be referred to herein as the "inactive" or "indifferent" electrode reservoir.
Systems of particular interest to the present application are "closed" reservoir systems. These are systems in which the active electrode reservoir is not replenished during operation, by a remote source of electrolyte solution. Thus, changes in reservoir content during electrotransport will generally be those due to the electrode operation (in addition to diffusion).
During many conventional electrotransport processes, ionic species, in addition to the charged drug species or therapeutic species to be transported, are generated or provided at the active electrode. For example, if the active electrode is the anode, and it is formed from a metal oxidizable under the operating potentials of the system, it will serve as a source of metal cations corresponding to the material from which the electrode is made. Also, again as an example, hydronium ion content (i.e. pH) may change during operation of certain electrodes (e.g., platinum, glass carbon or stainless steel electrodes).
During iontophoresis, since the therapeutic agent(s) is charged, it must compete with other similarly charged ions in the reservoir, for electrotransport through the skin and into the patient under the electromotive force of the applied potential. For example, if the active electrode is the anode, and the drug is to be delivered in a positively charged form, the positively charged drug must compete for transport with all other positively charged species in the reservoir or formed during the operation of the electrode and allowed to remain in solution in the reservoir. It follows, then, that for a constant current, efficiency of transport of the desired drug species across the skin membrane is reduced, if the operation of the active electrode involves generation of (or motivation of) competing species in the active electrode reservoir. This observation will hold whether the active electrode is the anode or the cathode.
Herein species in the active electrode reservoir similarly charged to the selected species for electrotransport (or transport) by iontophoresis (i.e. similarly charged to the target or therapeutic ions T.sub.i) will be referred to as "extraneous" ions (X.sub.i). For example, if the drug species to be selectively transferred is a positively charged species, all other cations in the active electrode reservoir will be referred to as "extraneous" ions or "extraneous cations". Alternatively, if the drug or treatment species to be transferred across the skin is negatively charged, all other anionic species in the active electrode reservoir will be referred to as "extraneous ions" or "extraneous anions". In general, the presence of extraneous ions reduces the efficiency of transport of a selected (i.e. target or therapeutic) ion, for a given iontophoresis system.
Alternatively, extraneous ions may be defined as those ions which will be transported from an active electrode reservoir, under applied potential, other than therapeutic species (if the therapeutic species is charged). That is, if the therapeutic species is uncharged, extraneous ions will be those ions which are delivered from the reservoir under the applied potential, during operation of the system.
Herein the term "target species", or "therapeutic species" and variants thereof refer in general to the agent to be selectively transported into subject for example by application of potential, whether that species is charged or not. Herein the terms "target ion", "therapeutic ion" and variants thereof refer to the particular ion species to be delivered by the iontophoresis process for therapy. In many instances the target ion will be a drug ion, or a selected metal ion. These species need not be the precise therapeutic agent(s) which operate in the body of the subject. They could, for example, be precursors to such agents. The terms are also intended to include within their scope ions delivered for purposes other than to treat some condition, for example to facilitate diagnoses. Thus, herein the term "therapy" and variants thereof is meant to include treatments of conditions, diagnostic procedures and other processes of medicine wherein an agent is delivered to a subject.
Methods have been developed to generate relatively extraneous ion free, or reduced extraneous ion concentration, systems. See for example the methods and apparatus described in U.S. Pat. Nos. 4,747,819 and 4,744,787 to Phipps et al., incorporated herein by reference and assigned to Medtronic, Inc., Minneapolis, Minn., the assignee of the instant invention. A basic principle of these methods is that the active electrode and/or components of the active electrode reservoir are selected such that electrochemical reactions conducted at the active electrode during operation provide species which do not interfere with or compete with the selected ionic species for transport (i.e. the target or therapeutic ion species). For example, if the drug to be delivered is positively charged, and it exists in the reservoir as a hydrochloride salt, the active electrode will be the anode. If a silver (or silver/silver chloride) electrode were used as the anode, then during operation of the electrode, a positive ion species formed at the anode would be silver cations. The reservoir includes chloride ions in solution from the hydrochloride salt of the drug, so silver chloride (which is insoluble) would be continuously formed during electrode operation. The silver chloride would precipitate from solution, at the surface of the active electrode. The result, then, would be continuous operation of the electrode to provide electromotive force to the cationic drug ion, without addition of positively charged species, (i.e., silver cations as extraneous cations)to the anodic reservoir in a mobile form. Thus, the concentration of extraneous ions in the active electrode reservoir is maintained at a minimum, or at least is not increased through operation of the system.
It is not, in practice, practical to completely exclude extraneous ion from typical electrotransport systems. The reasons for this include the fact that the hydrophilic reservoirs (typically aqueous systems involving gels or gel matrices) often may include therein, in addition to the drug species to be delivered (or a precursor for the drug species to be delivered) buffers, antibacterial agents, etc. Further, it may just be impractical in many instances to provide for a complete absence of ions (other than any ions to be selectively transported) and complete suppression of formation of such ions during electrode operation. Thus, even if the methods of Phipps et al. '787 and '819 are practiced to avoid introduction of more extraneous ions into a system, typical iontophoresis systems will in general include, ab initio, a significant concentration of extraneous ions. As will be seen in discussions below, this concentration of extraneous ions can have a significant effect on the performance of the iontophoresis process. In some instances it will negatively effect the process.